This invention relates to the field of on-line monitoring and diagnosis of motor-operated valves ("MOVs"). The invention further relates to mechanical and electrical measurements and signal processing.
Motor-operated valves are widely used in industry to control fluid-circulating systems for both liquids and gasses. In nuclear power plants, these fluid-circulating systems are especially crucial for safety, and a loss of control of these systems can have catastrophic consequences. Therefore, particular concern is placed on the reliability of MOVs. MOVs often operate in harsh environments and, consequently, they are subject to wear and prone to failure. The harsh operational environments make it difficult to monitor their operation in order to detect and diagnose failures at an incipient stage. The Nuclear Regulatory Commission ("NRC") has mandated that nuclear plants conduct a rigorous testing program on MOVs. Appropriate tests may range from daily operational checks to periodic testing. In the latter, the valve is heavily instrumented and forced to operate under emergency, steady-state and transient conditions. While these tests are extensive and provide much information concerning the operation of MOVs, they all suffer from the same drawback. They yield failure information after a failure has occurred, but do not predict future failures. Also to carry out these tests is very expensive.
Many efforts have been made to analyze MOV failure modes and to develop monitoring schemes that will generate information concerning the operation of MOVs. These schemes generally involve recognition of failure patterns in motor current and mechanical loading signatures using strain gages. Typically, the strain gauges are mounted on the valve stem or plug. See generally, H. D. Haynes and R. C. Kryter, "Condition Monitoring of Machinery Using Motor Current Signature Analysis", Oak Ridge National Laboratory; G. B. Kliman and J. Stein, "Induction Motor Fault Detection Via Passive Current Monitoring--A Brief Survey", Current Practices and Trends in Mechanical Failure Prevention--Proceedings of the 44th Meeting of the Mechanical Failure Prevention Group of the Vibration Institute, pp. 97-109, Virginia Beach, Va., 3-5 Apr. 1990; and S. Salter, "Valve Diagnostic Using Stem Load Measurements", Proceedings of the Institution of Mechanical Engineers International Conference on Valves for Power Plant Development and Testing, pp. 131-137, University of Southampton, 4-6 Apr. 1989. Several such systems have been marketed, meeting with varying degrees of commercial success. The existing systems and techniques do not fulfill all of the industry needs. Nuclear utilities and equipment vendors have pointed to opportunities for improvement in the area of fault detection and diagnosis in MOV systems.
MOVs are also used extensively in chemical manufacturing and in manufacturing processes where chemicals are used. Although these industries need not comply with NRC regulations they rely on the MOVs to properly regulate operations. Therefore there is also a need for evaluation of the operational integrity of MOVs in non-nuclear industries.
FIGS. 2 and 3 show a typical MOV in a schematic diagram and an exploded view, respectively. The figures show a model SMB-000, available from Limitorque, Inc. of Lynchburg, Va., which is of a typical design. A preferred embodiment of the invention will be discussed below in connection with such a typical MOV used in nuclear facilities. However, it will be understood that the invention may also be used with MOVs used in chemical processing situations, as well as with MOVs that are not configured the same as the MOV discussed.
A valve 201, consisting of a stem 224, a seat 203 and a plug 202 is driven by a motor 204, which is bolted to the actuator gear box or housing (not shown). The gear train between the output of the motor shaft 212 and the valve stem nut 220 is typically referred to as an operator, or an actuator. A pinion gear 206, which is attached to the motor shaft 212, drives a helical gear 207. The helical gear 207 drives a worm 210 that is connected to the splined, opposite end of the worm shaft 214.
This worm 210 is capable of moving axially as it rotates with the worm shaft 214. The axial movement of the worm is a means of controlling the torque that is applied to the valve stem nut 220. The worm 210 drives a lugged worm gear 216, which rotates the drive assembly 218. (The worm is a generally elongated gear apparatus. The worm gear 216 rotates about an axis that is perpendicular to the long axis of the worm (about which the worm rotates), and generally meshes with a portion of the length of the worm. The two are distinct.)
As the drive sleeve 222 rotates, the stem nut 220 raises or lowers a valve stem 224, which is connected to the stem nut through threads. When resistance arises to rotation of the worm gear 216, (because the valve plug 202 is beginning to seat or become obstructed), the worm 216 then begins to slide axially along its splined shaft 214, compressing a spring pack 226. (The worm 210 begins to slide before the worm gear 216 comes to a full stop.) This axial movement operates a torque limit switch 228 connected through leads 230 to a control circuit (not shown), causing the motor 204 to be de-energized when the spring pack is compressed to a preselected amount.
When the motor 204 is reversed, the worm gear 216 rotates approximately 130 degrees until its worm gear lugs 304 (FIG. 3) engage the drive sleeve 222. This motion, which is referred to as the "lost motion," allows the motor 204 to build up speed under no load conditions prior to engaging the drive sleeve 222. The engagement of the drive sleeve 222 in this fashion is rather abrupt and gives rise to what is referred to as a "hammer-blow effect".
The torque switch 228 measures the axial compression of the spring pack 226. Since the spring pack compression force is proportional to the worm thrust, the axial compression of the spring pack is a direct measure of the MOV output torque and an indirect measure of the MOV output thrust applied to the valve 202. Problems often arise with establishing and maintaining the proper setting of this torque switch.
There are normally two different switches that control an MOV: the torque switch and a geared travel limit switch 234. The geared travel limit switch assembly is driven directly by the operator gear train 208 and can be adjusted to operate at any point along the valve stroke. On a typical rising stem valve, the geared travel limit switch is used to de-energize the motor when the valve reaches its open position, prior to contacting the valve back seat. It is also difficult to establish and maintain the proper settings for this geared travel limit switch. As is mentioned above, MOV's also include an electric motor. For many reasons, it is useful to know the torque output by the motor. It is typically not possible to measure this torque without disrupting the normal operation of the MOV and motor.